1. Field of the Invention
The present invention relates to a plane light source unit that can form a reflection type liquid-crystal display device which is excellent in light utilizing efficiency, which is excellent in brightness and which is easy to view.
The present application is based on Japanese Patent Application No. Hei. 11-302619, which is incorporated herein by reference.
2. Description of the Related Art
A plane light source unit which permits a reflection type liquid-crystal display device to be viewed in a dark place, or the like, has been in great demand. In the meantime, the inventor of the present invention has tried to apply a front-lighting system in which a back-lighting type plane light source unit to be used in a transmission type liquid-crystal display device was disposed on the viewing side of a liquid-crystal cell. Such a back-lighting type plane light source unit uses a light pipe provided with a light output means so that light incident on a side surface exits from one of the upper and lower surfaces through the light output means. In the front-lighting system, the contents of display are viewed through the light pipe.
In the background-art plane light source unit using such a back-lighting type light pipe, however, there was a problem that it was difficult to put the plane light source unit into practical use because of shortage of clarity owing to remarkable disorder of a display image through the light pipe in addition to shortage of contrast at the time of switching-on of the plane light source unit and shortage of brightness at the time of switching-off of the plane light source unit. The shortage of contrast and the disorder of display were mainly caused by scattering of display light through a diffusing layer, or the like.
In consideration of the above description, the inventor of the present invention produced a plane light source unit which can suppress production of scattered light, and tried to prevent shortage of contrast and disorder of the display by applying the plane light source unit to a front-lighting system. In this case, however, a difference between brightness and darkness occurred in the plane light source because a luminance leveling effect owing to scattering of light was lost. Hence, it was found that dark portions were formed to cause a problem of missing portions in a display image.